[TowerTalk] Water tower omni

[bthorson at 4smps.com]

Unfortunately, I've got to deal with fixed stations, not mobiles. If 
there were a lot of random (time-dependent) scatter, it would probably 
work out the same way, but I can't be assured of that.

Roger sent me an interesting article on tangentially fired Yagis off of 
a tripod base that showed fairly uniform coverage, but I might have to 
mount the thing on the tower legs which would provide for some 
interesting cable routing. Alternatively, I could mount them on some 
horizontal booms from the catwalk, but that would expose them to more 
risk as well.

Using some type of array on top of the tank with a lightning rod in the 
middle might be the most cost effective. But, of course, there's that 
height thing where some have mentioned that once you get significantly 
elevated, the strike might just as well hit to the side.

Ben



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> Subject:
> Re: [TowerTalk] Water tower omni
> From:
> "Mike Clarson" <mclarson at rcc.com>
> Date:
> Wed, 5 Jul 2006 16:07:17 -0400
> To:
> <towertalk at contesting.com>
>
> To:
> <towertalk at contesting.com>
>
>
> While I cannot offer much as to how a lower antenna will be protected from
> lightning, I can offer some suggestions on constructing an "around the tank"
> antenna. In this type of array, phasing of the antennas is NOT important
> unless one is relying on electrical line lengths to split the power. With
> antennas spaced far apart as on a water tower, the phase relationship
> between the mobile and each antenna is always changing, so by phasing the
> lines to coordinate the launch phase doesn't make any difference as to how
> the array works. Years ago I was involved in designing a master antenna
> system to go on the side of a building, and we used 60 degree beamwidth
> panels center mounted on each wall, fed via a four way power splitter. An
> antenna manufacturer modeled it for us, (they built a scale model of the
> building AND the antennas and tested at a scaled higher frequency) and their
> conclusion was that it was a very complex pattern with many nulls, HOWEVER
> most of the nulls would interact with traditional mobile (spatial fading)
> flutter producing a pattern that was essentially omni. Well, kind of square
> actually because of the 60 degree antennas. The mobile flutter would
> interact with the interference pattern of the antennas reducing the nulls!
> Further, they moved the antennas to the corners so that two were mounted at
> right angles at opposite corners of the building (4 antennas total) and the
> pattern was nearly identical. You will have to take the hit of a four way
> power split (6 dB) on the gain. That means if you used 4  3 element Yagis (6
> dBd gain, 90 deg BW nominal) you'd end up with about a 0 dBd gain omni. If
> you used 5 element Yagis (9 dBd gain, 60 deg BW), you'd have about 3 dBd
> gain, but pattern wouldn't be quite as omni. Now, we never actually built
> the thing, so I have no first hand on the air experience. Not having to
> worry about phasing and, being able to mount the antennas in pairs may make
> it worth giving it a try. If you do, please report back.--Mike, WV2ZOW 
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